In light of the problem of ozone layer depletion which is of note in recent years, the restrictions on CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons) that are used as refrigerants in conventional refrigerating machines have become more stringent, and HFCs (hydrofluorocarbons) are coming into use as substitute refrigerants.
Mineral oils or hydrocarbon oils such as alkylbenzene are preferably used as refrigerating machine oils when CFCs or HCFCs are the refrigerants, but since changing the refrigerant can cause the refrigerating machine oil used with it to exhibit unpredictable behavior in terms of its compatibility with the refrigerant, lubricity, dissolved viscosity with the refrigerant and thermal and chemical stability, it has been necessary to develop different refrigerating machine oils for different refrigerants. Examples of refrigerating machine oils that have been developed for HFC refrigerants include polyalkylene glycols (see Patent document 1), esters (see Patent document 2), carbonic acid esters (see Patent document 3) and polyvinyl ethers (see Patent document 4).
HFC-134a is an HFC refrigerant that is routinely used as a refrigerant for automobile air conditioners, and while it has an ozone depletion potential (ODP) of zero, its high global warming potential (GWP) has led to its restriction in Europe. It has therefore become an urgent issue to develop refrigerants that can be used as substitutes for HFC-134a.
In light of this background it has been proposed to use, as substitute refrigerants for HFC-134a, fluoropropene refrigerants which have very low ODP and GWP, are noncombustible and are comparable or superior to HFC-134a in terms of their thermodynamic properties, as a measure of refrigerant performance. There has also been proposed the use of refrigerant mixtures of fluoropropene with saturated hydrofluorocarbons, C3-C5 saturated hydrocarbons, dimethyl ether, carbon dioxide, bis(trifluoromethyl)sulfide or trifluoroiodomethane (see Patent document 5).
In addition, there have been proposed refrigerating machine oils that employ mineral oils, alkylbenzenes, poly α-olefins, polyalkyleneglycols, monoesters, diesters, polyol esters, phthalic acid esters, alkyl ethers, ketones, carbonic acid esters, polyvinyl ethers and the like, as refrigerating machine oils that can be used with fluoropropene refrigerants or refrigerant mixtures of fluoropropene and saturated hydrofluorocarbons, C3-C5 saturated hydrocarbons, dimethyl ether, carbon dioxide, bis(trifluoromethyl)sulfide or trifluoroiodomethane (see Patent documents 5-7).    [Patent document 1] Japanese Unexamined Patent Publication HEI No. 02-242888    [Patent document 2] Japanese Unexamined Patent Publication HEI No. 03-200895    [Patent document 3] Japanese Unexamined Patent Publication HEI No. 03-217495    [Patent document 4] Japanese Unexamined Patent Publication HEI No. 06-128578    [Patent document 5] International Patent Publication No. WO2006/094303    [Patent document 6] Japanese Patent Public Inspection No. 2006-512426    [Patent document 7] International Patent Publication No. WO2005/103190